1. Field of the Invention
The present invention relates to a compound semiconductor device more particularly it relates to a gate electrode of a GaAs Schottky gate field effect transistor (GaAs MESFET).
2. Description of the Related Art
A III-V group compound semiconductor, for example, a GaAs crystal, has a band gap of 1.43 eV at room temperature, which is larger than that of silicon, and has a high resistivity of 10.sup.8 .OMEGA..multidot.cm or more. When the GaAs is used as a substrate of a single device and an integrated circuit, the parasitic capacitance is decreased and the elements are easily isolated from each other. Accordingly, GaAs is widely used as a transistor, particularly, a MESFET (Metal Semiconductor Field Effect Transistor), HEMT (High Electron Mobility Transistor) or a Schottky diode.
FIG. 1 shows a cross sectional view of a conventional GaAs MESFET, which comprises a semi-insulating GaAs substrate 1, an n-type GaAs active layer 2 formed on the GaAs substrate 1, a source electrode 3, a drain electrode 4, and a gate electrode 5.
To enhance the barrier height between the gate electrode 5 and the n-type GaAs active layer 2, an amorphous Si-Ge-B is conventionally used as a gate electrode material. The barrier height therebetween is about 1.0 eV. However, the Schottky junction between the amorphous Si-Ge-B layer and the GaAs active layer is unstable, and therefore, a threshold voltage (V.sub.th) of the FET after annealing is lowered with an elapse of time, as shown in FIG. 5.
The reason why the Schottky junction becomes unstable is that the Ge in the amorphous Si-Ge-B layer is easily reacted with GaAs.